The present invention relates to a mixing system especially designed to improve the mixing of a fluid with the exhaust gases of thermal engine, while also preventing the solid deposits of said fluid. The present invention can be used for example in an exhaust mixing chamber of a diesel engine wherein an aqueous solution of urea or fuel is injected in view of an after-treatment of the exhaust gases.
Exhaust gases formed in the combustion of fuel in an internal combustion engine may contain a proportion of undesirable substances such as nitrogen oxides (NOx), carbon monoxide (CO), un-burnt hydrocarbons (HC), soot, etc. . . .
To reduce air pollution, vehicles are therefore equipped with various after-treatment systems that deal with undesirable substances in exhaust gases.
A common exhaust gases after-treatment is a so called selective catalytic reduction (SCR). Exhaust gases wherein ammonia is added as a reducer is treated in a specific catalytic converter where nitrogen oxides are converted into water and nitrogen which are both non toxic substances.
Ammonia is introduced in the form of urea in an aqueous solution from which ammonia is obtained through hydrolysis. Urea is usually nebulised in the exhaust gas upstream of the catalytic converter. To this end, a urea injection nozzle is fitted on the exhaust line upstream from the catalytic converter.
A problem with this type of exhaust gases treatment is that, before it has transformed into ammonia, urea can crystallize. In concrete terms, the aqueous solution of urea which is sprayed through the nozzle inside the exhaust pipe, according to a direction which is angled with respect to the exhaust gases flow direction, tends to form a solid deposit on the exhaust pipe wall, on the internal side thereof, for example opposite of the injection point.
The consequence is that the cross section of the exhaust pipe is progressively reduced, which makes the engine efficiency decrease and which can seriously impair the engine operation in the long term.
Another example of an exhaust after-treatment system comprises a diesel particle filter which removes un-burnt particles contained in the exhaust gases. Such filter may eventually become clogged with the particles and needs to be regenerated from time to time. For promoting regeneration, it is often provided that fuel is injected in the exhaust line and oxidized to increase the exhaust gas temperature up to a point where the particles trapped in filter are oxidized. In this case also, there may appear problems related to the condensation of the fuel.
In both cases, there is, in addition to the problem of clogging or spoiling of the exhaust pipe by the fluid, the problem that the fluid which is not vaporized is then not available for its intended purpose, which can then affect the efficiency of the after-treatment or regeneration process. It is known from document FR-2.900.439 a system where the fluid is injected on an evaporating device located in the exhaust line. Nevertheless, it has proved difficult to design such a system which operates effectively under all engine operating conditions, especially due to the variations in the exhaust gases flow rate. It therefore appears that there is room for improvement in the systems for injecting a fluid in a mixing chamber carrying exhaust gases and mixing them.
It is desirable to provide an improved mixing system which can overcome the drawbacks encountered in conventional mixing systems, and particularly which prevents or at least limits the injected fluid from forming a deposit onto the mixing chamber surface.
An aspect of the invention concerns a mixing system for an exhaust gases after-treatment arrangement, said mixing system comprising:                a mixing chamber (2) in which exhaust gases can flow in a flow direction (FD);        a nozzle (5) designed to inject a fluid inside the mixing chamber (2), from an injection inlet (4) arranged in the mixing chamber wall, according to an injection direction (ID);        an evaporating device (8) positioned inside said mixing chamber (2), downstream from the injection inlet (4);        characterized in that the nozzle (5) and the evaporating device (8) are movable the one with respect to the other, so that a substantial portion of the flow of the fluid injected into said mixing chamber (2) hits the evaporating device (8) regardless of the flow rate of the exhaust gases        
Thus, in the mixing system according to the invention, the fluid most predominantly hits the evaporating device rather than the mixing chamber wall, whatever the flow rate of the first fluid—in the range of the normal operating conditions. As a result, substantially no or very little solid deposit can form on the mixing chamber wall, and clogging of said mixing chamber is prevented. Moreover, with such a mixing system, the evaporating device causes the fluid to be evaporated, which prevents it from crystallizing on the evaporating device and clogging it. Preferably, the evaporating device is designed to generate turbulence, to enhance the mixing of the injected fluid with the exhaust gases.
Alternatively, the turbulence generating device, or mixing device, can be a separate element which can be positioned either upstream or downstream from the injection inlet. In an implementation of the invention, said evaporating device can comprise a plate substantially orthogonal to the mixing chamber axis and provided with holes for allowing the passage of fluids there through.
Preferably, the evaporating device includes a thermally conductive material. The mixing system according to the invention may further comprise control means capable of controlling the position of the nozzle and/or of the evaporating device relative to the evaporating device or to the nozzle, respectively.
According to other possible features:                the control means comprise passive means including the thrust of the exhaust gases acting on a movable member for moving the evaporating device and/or the nozzle;        the control means comprise active means including an actuator acting on a movable member for moving the evaporating device and/or the nozzle;        the control means comprise biasing means for resisting and counteracting the thrust of the exhaust gases or the actuator;        the evaporating device is designed to be moved downstream within said mixing chamber when the exhaust gases flow rate increases; — the evaporating device is designed to be moved downstream within said mixing chamber, against the biasing means, under the effect of the exhaust gases thrust, the biasing means aiming at moving said evaporating device upstream with respect to said mixing chamber; with this arrangement, the position of the evaporating device with respect to the nozzle is automatically adjusted as a function of the exhaust gases flow rate;        the evaporating device comprises an inner duct located inside said mixing chamber and having an axis substantially parallel to the mixing chamber axis, and comprises a principal evaporating portion extending substantially across the inner duct; —a substantially cylindrical space is provided between the inner duct and the mixing chamber to allow a small amount of exhaust gases to flow through said space; this ensures that the inner duct can be easily heated by the exhaust gases, when they are hot.        the inner duct is designed to be moved downstream within said mixing chamber under the effect of the exhaust gases thrust, the biasing means aiming at moving said evaporating device upstream with respect to said mixing chamber;        the inner duct has an annular collar projecting inwardly from the upstream edge of said inner duct, the thrust of the exhaust gases applying on said annular collar;        an opening is provided in the inner duct wall to allow the injection of the fluid inside the inner duct through said opening;        the nozzle can pivot with respect to the mixing chamber about an axis which is parallel to a plane substantially tangential to said mixing chamber at the injection inlet and which is orthogonal to the mixing chamber axis so that the flow direction and the injection direction form a varying angle (α);        the control means are adapted to pivot the nozzle so that the angle (α) between the flow direction and the injection direction increases when the exhaust gases flow rate increases; In such case, the tilted position of the nozzle can then be adjusted so that the injected fluid, drawn by the exhaust gases, is directed towards the evaporating device.        the fluid comprises an aqueous solution of urea, or fuel; and        said evaporating device includes a thermally conductive material.        
These and other features and advantages will become apparent upon reading the following description in view of the drawings attached hereto representing, as non-limiting examples, embodiments of a mixing system according to the invention.